In situ transmission electron microscopy observation of the conversion mechanism of Fe2O3/graphene anode during lithiation-delithiation processes.

Transition metal oxides have attracted tremendous attention as anode materials for lithium ion batteries (LIBs) recently. However, their electrochemical processes and fundamental mechanisms remain unclear. Here we report the direct observation of the dynamic behaviors and the conversion mechanism of Fe2O3/graphene in LIBs by in situ transmission electron microscopy (TEM). Upon lithiation, the Fe2O3 nanoparticles showed obvious volume expansion and morphological changes, and the surfaces of the electrode were covered by a nanocrystalline Li2O layer. Single-crystalline Fe2O3 nanoparticles were found to transform to multicrystalline nanoparticles consisting of many Fe nanograins embedded in Li2O matrix. Surprisingly, the delithiated product was not Fe2O3 but FeO, accounting for the irreversible electrochemical process and the large capacity fading of the anode material in the first cycle. The charge-discharge processes of Fe2O3 in LIBs are different from previously recognized mechanism, and are found to be a fully reversible electrochemical phase conversion between Fe and FeO nanograins accompanying the formation and disappearance of the Li2O layer. The macroscopic electrochemical performance of Fe2O3/graphene was further correlated with the microcosmic in situ TEM results.